Tunable Cellulose – Keratin Biofibers: Balancing Strength and Sustainability

Cellulose-keratin biofibers were regenerated using a novel ethylenediamine (ED)/potassium thiocyanate (KSCN) solvent system, which co-dissolves both polymers at 90°C within 2–3 h. Both components are inexpensive commodity chemicals, recoverable in principle by distillation (ED) and aqueous extraction (KSCN), unlike the high-boiling ionic liquids whose costly recycling remains a barrier to scale-up. Continuous fibers containing 0 to 40 wt% keratin were produced by dry-jet wet spinning, with no discrete protein domains resolved at the magnifications examined. Pure regenerated cellulose exhibited a Cellulose III crystal structure, whereas keratin incorporation induced a transition to Cellulose II with progressively lower crystallinity and molecular orientation. These changes governed fiber properties: tenacity decreased from 1.61 ± 0.10 to 0.69 ± 0.06 g/den, initial modulus from 80.7 ± 9.2 to 34.8 ± 15.0 g/den, and birefringence from 0.036 to 0.022, while elongation at break rose from 9.3 ± 0.8% to 14.4 ± 2.8%. After 30 days of aerobic composting, fibers with higher keratin loadings showed the largest reductions in peak load and elongation at peak load, indicating a compositionally tunable balance between durability and compost-induced degradation. ED/KSCN-regenerated fibers are therefore promising for sustainable applications requiring tunable ductility and controlled loss of mechanical integrity at end of life.